swift234

Fix enum instruction simplification.

When simplifying an 'enum' instruction that appears in a block that is
the destination of a switch_enum, we need to ensure that the block with
the 'enum' instruction is uniquely chosen based on the same enum tag
that the 'enum' instruction uses, not just that we'll reach that
destination block with the same enum tag (since we could have multiple
tags target the same block, and could have a default case).

Fixes rdar://problem/18330952.

Swift SVN r22065